1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and more particularly, to such a method of manufacturing a semiconductor device as having a step of etching an underlying layer by using a photo resist film as a mask. The present invention further relates to a method of forming a photo-mask having a pattern such as a hole which is to be transferred to a photo resist film that is used as a mask for etching an underlying layer. The present application is based on Japanese Patent Application No. 2001-185976, which is incorporated herein by reference.
2. Description of the Related Art
In the lithography step, which is one of the steps of a process for manufacturing semiconductor devices, a resist film is applied to an underlying layer formed on a semiconductor substrate, and the exposure and development are performed on the resist film by use of a photo-mask to form a resist pattern (in other words, the pattern of the photo-mask being transferred to the resist layer), followed by the etching or impurity introduction being performed on the underlying layer by use of the resist pattern as a mask.
More detailed description will be made on the so-called dual-damascene interconnection structure of a trench first process type, which is taken as an example, with reference to FIGS. 5A to 5D. The dual-damascene interconnect having a trench interconnect (wiring) formed in a trench of an interlayer dielectric layer and a via hole electrically connecting the trench interconnect to a conductive layer (or an impurity region) formed under the interlayer dielectric layer. As shown in FIG. 5A, an interlayer dielectric 103 is formed with a prescribed film thickness on a semiconductor substrate 101 in which a conductive layer 102 of the underlying layer is formed. Next, as shown in FIG. 5B, a first resist film 104 is applied and by performing the exposure and development of the first resist film 104 by use of a first photo mask 105 having a mask pattern corresponding to an interconnect trench, a first resist pattern 106 of pattern shape corresponding to a trench inter connect is formed. After that, the above-described interlayer dielectric 103 is selectively etched to a prescribed thickness by use of this first resist pattern 106 as a mask and an interconnect trench 107 is formed. Subsequently, as shown in FIG. 5C, a second resist film 108 is applied to the whole surface and by performing the exposure and development of the second resist film 108 by use of a second photo mask 109 having a via pattern, a second resist pattern 110 of pattern shape corresponding to a via is formed in the above-described interconnect trench 107. After that, by use of this second resist pattern 110 as a mask the above-described interlayer dielectric 103 is selectively etched until the conductive layer 102 of the underlying layer is reached, whereby a via hole 111 that opens the conductive layer 102 of the underlying layer is formed. In addition, as shown in FIG. 5D, by filling the above-described interconnect trench 107 and via hole 111 with a conductive material 112, a trench interconnect 113 is formed in the interconnect trench 107 and a via 114 that electrically connects the trench interconnect 113 and the conductive layer 102 of the underlying layer is formed in the via hole 111.
In such process as described above, since the second resist film 108 is coated after forming the interconnect trench 107, the second resist film 108 is not formed with a uniform thickness, but formed such that its thickness over the inner (or central) portion of the interconnect trench 107 is smaller than that over the outer (or edge) portion of the trench 107. The inventor of the present invention has revealed that the difference in thickness of the photo resist film 18 over the trench 107 causes problems as described below.
Specifically, a via hole is not always formed in substantially the middle of an interconnect trench, and it is formed at a various kind of positions based on where airing layer or an impurity region to which the via hole should be connected is formed. For example, as shown in FIG. 6A, a via hole 111A is opened in a position near the center in the width direction of a trench interconnect 113 and a via hole 111B is opened in a position near an edge, as shown in FIG. 6B. As described above, the second resist film 108 is not formed with the uniform thickness into the trench interconnect 113, but the second resist film 108 is formed with a film thickness in the region of both edge sides of an interconnect trench 107 being larger than that in the region of the middle of the interconnect trench 107, as shown in FIG. 6B. For this reason, if a pattern of the photo mask 109 having a via hole pattern 111A and 111B is transferred to the second resist film 108 to made a second resist pattern 110 for forming a via hole is formed, the resist pattern 110 is formed to have a pattern opening 110A having substantially the same size as a via hole pattern 111A. However, on the other hand, the resist pattern 110 is formed to have a pattern opening 110B having a tapered shape in the depth direction of the opening in the region on both sides of the interconnect trench 107 because the exposure at the bottom of the second resist film 108 decreases due to the resist film thickness being large in the region on both sides of the interconnect trench 107. With the result that although the size of the pattern opening 110B on the surface of the second resist film 108 is formed to have substantially the same size as the via hole pattern 111B, the size of the pattern opening 110B at the boundary between the second resist film 108 and an interlayer dielectric 103, i.e., on the bottom surface of the second resist film 108, becomes smaller than the via hole pattern 111B. Therefore, if a via hole is opened by etching the interlayer dielectric 103 by use of such a resist pattern as a mask, the diameter size of a via hole 121A formed in the middle region becomes almost as designed, however, the diameter size of a vie hole 121B formed in the edge region on both sides becomes smaller than a design size, thus a via resistance of the via hole 121B is larger than it is designed.
As described above, a size of a via hole transformed in substance from a photo mask to a photo resist film varies due to a lack of uniformity of a film thickness of the photo resist film, which is caused by a surface shapes i.e. a surface concavity and convexity (uneven surface) of an underlayer film due to steps of an underlying layer.
It should be noted that a size of a pattern that is actually transferred to a photo resist film is discussed in Japanese Patent Laid-Open No.2000-292903 (hereinafter, a referential document 1) and Japanese Patent Laid-Open No. 2001-85583 (hereinafter, a referential document 2). In the referential document 1, it is pointed out that the size of a pattern actually transferred to the photo resist film is affected by the difference in thickness of the photo resist film. However, the document 1 does not address at all about how and where the difference in thickness of the resist film is generated. Especially, it does not recognize that a surface concavity and convexity of an underlayer which is to be selectively etched by use of a photo resist film, causes variation in film thickness of that photo resist film. Rather, in accordance with the teachings of the referential document 1, a variation of the actually-transferred size of the pattern caused by the film thickness of a photo resist film is solved by providing an anti-reflection layer on and/or under the photo resist layer. In addition, the document 1 suggests that a variation of a size of a pattern that has been actually transferred on a photo resist film is caused by the layer structure of the underlayer film which is subjected to etching, rather than by the variation of the film thickness of a photo resist film, and based on this suggestion, the size of the pattern to be transferred to the photo resist film is controlled in accordance with the analyzing result of the layer structure of the underlayer film.
On the other hand, the referential document 2 discusses that a pattern which as been actually transferred on a photo resist film, especially the size of an opening, deviates from a designed value by the number of and/or etch size of other openings which are formed near the opening is question simultaneously with one another. However, it does not recognize that a surface concavity and convexity of an underlayer film which is to be selectively etched affects a variation of a film thickness of a photo resist film, either.
The method according to the first aspect of the present invention may be applied to transfer the pattern of a mask, which has a first pattern hole and a second pattern hole being different in size from each other, to a resist film covering an underlying layer having an uneven surface. Specifically, the underlying layer is at first covered with a resist film. The underlying layer has an uneven surface so that the resist film has first and second portions which are different in thickness from each other due to the uneven surface of the underlying layer. The pattern of the mask is then transferred to the resist film to thereby form a resist pattern film. In this step, the mask having, as its pattern, first and second pattern-holes to form first and second openings respectively in the first and second portions of the resist film, and the first and second pattern-holes of the mask are different in size from each other. Thereafter, etching is performed on the underlying layer by using the resist pattern film as a mask to make first and second holes in the underlying layer. Thus, the first and second holes which have been formed in the underlying layer become substantially the same in size as each other because of the fact that the first and second pattern-holes of the mask are different in shape from each other.
The method according to the second aspect of the present invention may be applied to transfer the pattern of a mask which has a hole for defining an exposed area of a resist film to a resist film having ununiform thickness, the exposed area containing a first portion and a second portion being larger in width than a first portion. Specifically, the underlying layer is at first covered with the resist film. The resist film has a first portion and a second portion that is larger in thickness than the first portion. The pattern of the mask is then transferred to the resist film to thereby form a resist pattern film. In this step, the mask has, as its pattern, a first hole for defining a first exposed area of the resist film, the first exposed area of the resist film containing respective parts of the first and second portions of the resist film, and the first and second portions are formed in such a manner that the part of the first portion of the resist film is smaller in width than the part of the second portion of the resist film. Thereafter, etching is performed on the underlying layer by using the resist pattern film as a mask to form a first opening in the underlying layer. Thus, the first opening is thereby formed with a width that is substantially equal to the width of the part of the first portion of the resist film.
In the method according to the third aspect of the present invention, if the exposed area of a resist film defined by the opening of a photo-mask is detected to cover respective part of a first portion of the resist film and a second portion of the resist film that is larger in thickness than the first portion, the opening is formed in such a manner that the part of the second portion of the resist film is larger in width than the part of the first portion of the resist film. Specifically, it is detected whether the exposed area of the resist film defined by the opening of the photo-mask is within the first portion of the resist film, covers respective parts of the first and second portions of the resist film, or is within the second portion of the resist film. The resist film has the first portion and the second portion that is larger in thickness than the first portion. If the exposed area defined by the opening is detected to cover respective parts of the first and second portions of the resist film, the opening is formed in such a manner that the part of the second portion of the resist film is larger in width than the part of the first portion of the resist film. The opening defines an exposed area of a resist film which is used as a mask film to selectively etching a layer underlying the resist film and to make the hole in the layer. Thus, the hole is formed in the layer with a width that is substantially equal to the width of the part of the first portion of the resist film.
As described above, a resist mask which is formed in the present invention may be free from dimensional errors of pattern caused by a difference in the resist film thickness, which is ascribed to the level-difference portion of the underlying layer. Furthermore, the present invention may provide a design method of the mask pattern for forming the resist mask.